1. Field of the Invention
This invention relates to AC electric energy meters of the watthour meter type, including voltage and current sensing transducers for applying signals responsive to the current and voltage components of an electric energy quantity to be measured by electronic measuring circuits, and more particularly to such meters including a mutual inductance current sensing transducer capable of sensing widely varying amplitudes of the current component and producing low level output signals suitable for use by the electronic measuring circuits.
2. Description of the Prior Art
Devices for AC electric energy measurement are extensively used by producers of electric energy for measuring consumption by separate energy users. Typically, watthour meters are used for indicating consumption in kilowatt-hours. The watthour meters are usually of the induction type having a rotating disc, which are recognized as having high degrees of reliability and accuracy, being available at reasonable costs, and being capable of outdoor operation under widely varying extremes of temperature and other ambient conditions.
It is also known to measure AC electric energy quantities such as kilowatt-hours, volt-ampere hours, reactive volt-ampere hours, with electronic measuring circuits. Typically, voltage and current instrument transformers provide signals proportional to the voltage and and current components of an electric energy quantity to be measured. Analog multiplier circuit arrangements are known in one type of measuring circuit and they are arranged to produce a signal proportional to the time integral of the product of the voltage and current components. One electronic measuring circuit is described in U.S. Pat. No. 3,764,908, assigned to the assignee of this invention, wherein voltage and current signals are applied to a semiconductor device having a logarithmic computing characteristic. An output signal is produced therefrom which is proportional to the product of the voltage and current signals and a measured value of the electric power quantity.
Another known analog multiplier type of AC electric energy measuring circuit is referred to as a time-division-multiplication type of measuring circuit. In U.S. Pat. No. 3,864,631, assigned to the assignee of this invention, the technique of analog multiplication is disclosed. A voltage component signal is sampled to derive a variable pulse width modulated signal corresponding to the voltage component variations. A current component signal is sampled at a rate responsive to the variable pulse width signals. A resultant output is produced, consisting of a series of pulses having amplitudes proportional to the instantaneous current values and pulse widths proportional to the instantaneous voltage values. The resultant pulse signals are filtered to obtain an average value, or DC level, proportional to measured AC electric power. The average value signal controls a voltage-to-frequency conversion circuit, utilizing integrating capacitors. Variable frequency pulses from the conversion circuit are totalized, so that a total pulse count is a measure of the electric energy consumption.
In U.S. Pat. No. 3,343,084, assigned to the assignee of the present invention, voltage and current components of an electric energy quantity to be measured are applied to a Hall generator. The output of the Hall generator provides a signal proportional to the product of the voltage and current signal inputs. The Hall generator output is applied to a saturable core transformer integrating device to produce pulses which are proportional to the time integral of the Hall generator output or of the electric energy to be measured. The voltage and current inputs to the Hall generator are applied from the detachable contact terminals of a detachable watthour meter.
In U.S. patent application Ser. No. 064,603, filed Aug. 7, 1979, now U.S. Pat. No. 4,255,707, which is a continuation of U.S. patent application Ser. No. 919,874, filed June 26, 1978, now abandoned, both assigned to the assignee of this invention, an AC electric energy measuring circuit and method is described wherein the voltage component of an electric energy quantity to be measured is converted by electric circuit techniques to a signal proportional to the time integral of the voltage component. The time integral voltage signal is compared to incremental reference levels. Each instance that a referenced level is reached, the instantaneous magnitude of the current component is sampled and converted to digital signals. These digital signals are summed to produce an output signal corresponding to a measure of AC electric energy in watthours. Some of the component drift disadvantages of prior analog multiplier circuits are avoided by the aforementioned circuit.
A further example of an electric energy measuring circuit is disclosed in U.S. Pat. No. 4,077,061, assigned to the assignee of this invention, where analog-to-digital sampling of the voltage and current components is performed for subsequent digital processing and calculation. A number of different electric energy parameters are calculated by digital computational circuit techniques.
In each of the aforementioned circuit techniques for electric energy measurements, the voltage and current inputs to the AC energy measuring circuit are provided directly by the line voltage and current or by instrument transformers for producing signals proportional to the line voltage and current components of the electric energy quantity being measured. Although electronic circuits are operable in small signal ranges, the electric power voltage and currents are several magnitudes larger. Thus, the sensing transducers which provide the voltage and current responsive inputs to the measuring circuits must have large transformation ratios. Also, the sensing transducer's response must be linear with the proportionalities between the input and output being constant. In the case of the current transducer, the linear response must be over a wide range of current values to be sensed.
In typical electric energy measurements at a utility customer location, sixty Hertz AC electric power is delivered at substantially constant line voltages of either one hundred-twenty or two hundred-forty volts defining the voltage components of the electric energy quantity to be measured. On the other hand, load currents which define the current component of the electric energy quantity to be measured vary considerably. In measuring operation, a substantially linear response is desired in a general range of from one-half ampere to two hundred amperes, or in a current variation ratio of approximately four hundred to one. With line current values above 200 amperes and below one-half ampere degradation of the linear response begins to occur in many systems. Accordingly, standard potential transformer arrangements can provide practical voltage sensing transducers. However, current transformers receiving the aforementioned substantially wide input variations, with a ratio in the order of four hundred to one, and producing low level signal outputs require arrangements which are often of substantial size and cost. When it is desired to manufacture electronic AC energy measuring circuits and devices which are relatively compact and comparable in cost to the aforementioned conventional induction type watthour meters, the voltage and current sensing transducers present substantial contributions to the overall size and cost of such devices. As is known in accurate current transformer transducers, the ampere turns of the primary and of the secondary must be equal, and since current levels can produce 400 ampere-turns in the primary, the secondary winding sizes become substantial in order to produce linear low level signal outputs resulting in current transformers that are bulky and are relatively costly.
Accordingly, it is desirable to provide the voltage and current sensing transducers for electronic AC energy measuring circuits which are highly reliable and accurate and are adapted for standard connection to the conductors carrying the electric energy quantity to be measured, such as provided at service entrance conductors at a residential electric power user's location. It is further desirable that the current sensing transducer of such devices be compact, capable of mass production by economical manufacturing techniques and be operable to produce low level signal outputs accurately in response to large variations of load currents to be sensed.